As well known in the art, the internal combustion engine inevitably emits exhaust gases. The components and quantities of the exhaust gases depend upon the running state of the internal combustion engine. In the ordinary tendencies, at a heavy loading time when a throttle opening is widened, the cleanness of the exhaust gases is liable to become lower along with the fuel economy. In recent years, on the other hand, the demand for the cleanness of the exhaust gases of the vehicle having the internal combustion engine mounted thereon has grown higher and higher, and a hybrid drive system has been developed to satisfy the demand.
This hybrid drive system is a drive system equipped with the internal combustion engine and an electric motor as its prime movers and is basically constructed such that the internal combustion engine is activated in the most efficient state whereas the electric motor is run as an assistant or a prime mover in other running states. Thus, the hybrid drive system is equipped with an electric motor capable of controlling its torque with an electric current so that it does not employ any transmission as in the prior art adopting the internal combustion engine exclusively as the prime mover. In the so-called "parallel hybrid type", however, the transmission has been mounted on the hybrid drive system in which the internal combustion engine is used not only for the power generating prime mover but also for a running prime mover. In addition, there has been developed a system in which the torque of the internal combustion engine and the torque of the electric motor are inputted to a single transmission mechanism such as a planetary gear mechanism so that the output torque of the internal combustion engine may be amplified and outputted.
An example of this system has been disclosed in Japanese Patent Laid-Open No. 37411/1997. As shown in FIG. 11 of the Laid-Open, the system is equipped with a double pinion type planetary gear mechanism which has: a ring gear coupled to an output shaft; a sun gear coupled to a motor/generator; an input clutch for coupling a carrier and an engine; a brake for fixing the carrier selectively; and an integral clutch for coupling the carrier and the sun gear to integrate the planetary gear mechanism as a whole. Moreover, the output shaft can be coupled to a continuously variable transmission.
In the hybrid drive system disclosed in FIG. 11 of the Laid-Open, therefore, when a motive power is inputted from the motor/generator to the sun gear with a motive power being inputted to the carrier from the engine, a higher torque than the engine torque is outputted to the ring gear acting as an output member. When the motor/generator is reversed to absorb the motive power, on the other hand, it is possible to generate an electric power. Moreover, the driving force can be continuously changed according to the gear ratio at the continuously variable transmission. By making use of the torque amplifying function in the planetary gear mechanism and the continuous gear ratio changing function of the continuously variable transmission, the internal combustion engine can be run to optimize the fuel economy.
If the electric motor is provided as the prime mover, as described above, the output torque of the electric motor can be controlled with the electric current so that the transmission can be basically eliminated. In the aforementioned hybrid drive system of the prior art, the planetary gear mechanism and the continuously variable transmission have been utilized as control means for the run of the internal combustion engine to optimize the fuel economy. In the aforementioned construction of the planetary gear mechanism, therefore, during the run with the output of the internal combustion engine, the ring gear acting as an output element cannot be rotated backward of the carrier acting as an input element. In the case of a reverse run by the aforementioned hybrid system of the prior art, therefore, the internal combustion engine is held in the idling state, and the carrier is fixed by the brake. In this state, the ring gear or the output element is rotated backward of the internal combustion engine by causing the motor/generator to function as a motor.
In short, the hybrid drive system of the prior art is constructed to effect the reverse run with the electric motor. When the state of charge (SOC) of the (storage) battery is short, therefore, there is no possibility that a sufficient torque demanded for the reverse run can be outputted. In order to eliminate this disadvantage, the internal combustion engine may be started to charge up the battery. Then, the reverse run has to await completion of the charge. Especially the parallel hybrid system in which the electric motor acts as a generator cannot perform the run and the generation simultaneously with the electric motor. Thus, there arises a drawback that the reverse run becomes difficult as soon as the charge in the battery drops.
The invention has an object to provide a hybrid drive system capable of retaining a torque for a reverse run even when the charge of the battery is short.
Another object of the invention is to provide a hybrid drive system capable of running backward with an internal combustion engine and having a small size.